This invention relates to treatment of aluminum surfaces. More particularly, it relates to electrochemical treatment of an aluminum surface to provide a surface thereon suitable for lithographic uses.
Hydrochloric acid electrolytes are commonly used to electrochemically treat aluminum surfaces to render them suitable for subsequent lithographic applications. British Patent Specification 831,998, for example, describes the electrolytical roughening of aluminum foil using a low concentration hydrochloric acid electrolyte for lithographic uses. British Patent Specifications 879,768 and 896,563 and U.S. Pat. Nos. 3,072,546 and 3,073,765 all describe the treatment of aluminum surfaces with hydrochloric acid while applying an alternating current to the aluminum plates to render the plates suitable for lithographic uses. While this treatment has been considered to be satisfactory, it apparently functions best with relatively high purity aluminum alloys such as an alloy having less than half of 1% impurity such as, for example, Aluminum Association Alloy 1145. In addition, it has been found that dissolved aluminum from the treatment, which apparently is in the nature of an etch, accumulates in the hydrochloric acid bath which, in turn, substantially decreases the treatment rate and severely limits the effective bath life.
When the conventional hydrochloric acid treatment is applied to less pure aluminum alloys such as, for example, Aluminum Association Alloy 1100 which may contain as much as 1% silicon plus iron as well as as much as 0.2% copper, the hydrochloric acid tends to preferentially etch certain portions of the metal surface (presumably representing one or more metallic impurities present in the alloy) resulting in a proliferation of such etch pits scattered randomly across the surface. These pits, while microscopic in nature, are much deeper than the desired uniform etching or roughening and therefore can interfere with the subsequent printing process when the surface is subsequently coated with a photosensitive resin such as used in normal lithographic processes.
It is also known to treat aluminum surfaces with certain acids or combinations of acids to achieve polishing effects rather than etching or roughening. For example, Shibasaki U.S. Pat. No. 3,365,380 and Shiga et al. U.S. Pat. Nos. 3,342,711 and 3,366,558, all assigned to Kyowa Hakko Kogyo Company, each refer to an electrolytic polishing effect obtained on aluminum and other metals using a mixture which may include sulfuric acid and gluconic acid.
The same assignee also disclosed in Shibasaki U.S. Pat. No. 3,389,066 the use of combinations of acids including hydrochloric acid and gluconic acid as an electrolytic polishing agent for iron or iron alloys.
It was, therefore, quite surprising to discover that aluminum surfaces, when treated in a special mixture of acids, are roughened more uniformly in a manner conducive to subsequent lithographic uses than was known or predictable from the prior art. Furthermore, such treatment enables the use of less pure alloys such as, for example, Aluminum Association Alloys 1100 and 3003, to be treated without the occurrence of the undesirable preferential deep etching previously discussed. Finally, the treatment apparently also provides a chelating effect to chemically complex the dissolved aluminum and other impurities which form in the bath thus extending the life of the bath.